From 7b36ca8c16ec62cb2171666535b0f91dfe44ffc1 Mon Sep 17 00:00:00 2001
From: Timothy Warren <twarren@nabancard.com>
Date: Mon, 18 Mar 2019 12:12:13 -0400
Subject: [PATCH] Complete step 6

---
 db.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 49 insertions(+), 5 deletions(-)

diff --git a/db.c b/db.c
index dea9694..ce61718 100644
--- a/db.c
+++ b/db.c
@@ -83,6 +83,13 @@ struct Table_t {
 };
 typedef struct Table_t Table;
 
+struct Cursor_t {
+    Table* table;
+    uint32_t row_num;
+    bool end_of_table; // Indicates a position one past the last element
+};
+typedef struct Cursor_t Cursor;
+
 InputBuffer* new_input_buffer() {
     InputBuffer* input_buffer = malloc(sizeof(InputBuffer));
 
@@ -242,6 +249,24 @@ void deserialize_row(void* source, Row* destination) {
     memcpy(&(destination->email), source + EMAIL_OFFSET, EMAIL_SIZE);
 }
 
+Cursor* table_start(Table* table) {
+    Cursor* cursor = malloc(sizeof(Cursor));
+    cursor->table = table;
+    cursor->row_num = 0;
+    cursor->end_of_table = (table->num_rows == 0);
+
+    return cursor;
+}
+
+Cursor* table_end(Table* table) {
+    Cursor* cursor = malloc(sizeof(Cursor));
+    cursor->table = table;
+    cursor->row_num = table->num_rows;
+    cursor->end_of_table = true;
+
+    return cursor;
+}
+
 void* get_page(Pager* pager, uint32_t page_num) {
     if (page_num > TABLE_MAX_PAGES) {
         printf("Tried to fetch page number out of bounds. %d > %d\n", page_num, TABLE_MAX_PAGES);
@@ -273,33 +298,50 @@ void* get_page(Pager* pager, uint32_t page_num) {
     return pager->pages[page_num];
 }
 
-void* row_slot(Table* table, uint32_t row_num) {
+void* cursor_value(Cursor* cursor) {
+    uint32_t row_num = cursor->row_num;
     uint32_t  page_num = row_num / ROWS_PER_PAGE;
-    void* page = get_page(table->pager, page_num);
+    void* page = get_page(cursor->table->pager, page_num);
     uint32_t row_offset = row_num % ROWS_PER_PAGE;
     uint32_t byte_offset = row_offset * ROW_SIZE;
     return page + byte_offset;
 }
 
+void cursor_advance(Cursor* cursor) {
+    cursor->row_num += 1;
+    if (cursor->row_num >= cursor->table->num_rows) {
+        cursor->end_of_table = true;
+    }
+}
+
 ExecuteResult execute_insert(Statement* statement, Table* table) {
     if (table->num_rows >= TABLE_MAX_ROWS) {
         return EXECUTE_TABLE_FULL;
     }
 
     Row* row_to_insert = &(statement->row_to_insert);
+    Cursor* cursor = table_end(table);
 
-    serialize_row(row_to_insert, row_slot(table, table->num_rows));
+    serialize_row(row_to_insert, cursor_value(cursor));
     table->num_rows += 1;
 
+    free(cursor);
+
     return EXECUTE_SUCCESS;
 }
 
 ExecuteResult execute_select(Statement* statement, Table* table) {
+    Cursor* cursor = table_start(table);
+
     Row row;
-    for (uint32_t i = 0; i < table->num_rows; i++) {
-        deserialize_row(row_slot(table, i), &row);
+    while (!(cursor->end_of_table)) {
+        deserialize_row(cursor_value(cursor), &row);
         print_row(&row);
+        cursor_advance(cursor);
     }
+
+    free(cursor);
+
     return EXECUTE_SUCCESS;
 }
 
@@ -335,6 +377,8 @@ Pager* pager_open(const char* filename) {
     return pager;
 }
 
+
+
 Table* db_open(const char* filename) {
     Pager* pager = pager_open(filename);
     uint32_t num_rows = pager->file_length / ROW_SIZE;